Physico-chemical properties of fruit and vegetable juices as affected by pulsed electric field: a review

ABSTRACT

Treatment of fruit and vegetable juices with pulsed electric field (PEF) process is one of the promising non-thermal technologies with the great potential for microbial and enzymes inactivation, shelf-life extension, and preservation of nutritional, vitamins, aroma compounds and sensorial attributes. The present review study summarized the influence of PEF treatment on the physical and chemical parameters, color change, bioactive compounds and sensorial attribute of apple, carrot, citrus, grape, peach, pomegranate, watermelon and tomato juices. The PEF treatment resulted increase of juice yield of pressing with lower non-enzymatic browning. PEF treatment can be applied in fruit and vegetable juices industry to improving physicochemical properties, rheological behavior and antioxidant capacity of juices. PEF processed juices retained more vitamins, ascorbic acid, carotenoid, anthocyanins, lycopene and organoleptic characteristics (appearance, flavor, taste, aftertaste, color and overall acceptance) than thermally processed juice; of course, in some juices, the sensorial attributes and physicochemical properties were not affected by the PEF treatment. High intensity pulsed electric field (HIPEF) process is efficient enough to inactivation of microorganisms and enzymes (pectinmethylesterase (PME), polyphenoloxidase and peroxidase) in fruit and vegetable juices at levels equivalent to heat treatment, consequently improved shelf life and quality of the preserved juices. The application of PEF can save a considerable amount of time and increase production efficiency. Therefore, PEF technology could be an alternative to thermal treatment to obtain fruit and vegetable juices with high nutritional quality and good sensorial attributes.

KEYWORDS:

• Bioactive compounds
• nutrition
• PEF
• sensorial attributes
• shelf life

Introduction

The food industry has shown an increased interest in the manufacture of healthier, high quality, minimally processed and more natural food products. The quality of fruit and vegetable juices is defined by their physical properties and enzymatic, microbiological, and sensory characteristics and stability.^[1–4] PEF constitutes a suitable substitute for thermal methods for inactivating enzymes and pathogenic microorganisms simultaneously retaining sensory and nutritional components of fruit and vegetable juices.^[5–7] Also, PEF-pretreatment noticeably accelerated diffusion even at low temperatures within 20–40 °С.^[8]

The application of PEF in food processing has been gaining interest as a non-thermal technology to inactivate microorganisms and enzymes by maintaining the nutritional quality, antioxidant content and freshness of juices .^[9] The process generates some amount of heat during its application but its maximum temperature (40 °C) is way below thermal processing temperatures. Fruit and vegetable juices are placed between two electrodes and are applied pulses with a high voltage (usually 50 kV/cm) for short periods of time (μs to ms).^[10] Its principle is a combination of electroporation and electropermeabilization.^[11] PEF process consists of the use of high external electric fields (1–50 kV/cm) for a short time (milliseconds (ms) to microseconds (μs)) to cell material inducing the electroporation of the cell membranes.^[7,9] Electroporation means an increase in the permeability of the cell membrane to the transport of ions and macromolecules. Therefore, the decrease of the resistance to diffusion through cell membrane makes the extraction of the bioactive compound from the cell easier, promoting the extraction yield.^[9] The effectiveness of PEF depends on important processing factors such as field strength, pulse width, pulse frequency, treatment time, polarity, temperature applied etc.^[12] Fruit and vegetable juices matrix is an important factor in deciding the efficiency of PEF process. Researchers found lesser effectiveness of PEF processing of juices containing high amount of macronutrients such as fats and proteins compared to simple microbial suspensions.^[12,13]

Electric conductivity of the medium is also an important parameter and PEF treatments are more effective in foods with low electrical conductivity. The difference between the electrical conductivity of the medium or juice and that of the microbial cytoplasm increases the flow of ionic substances across the membrane. As a result, the membrane is weakened and becomes more susceptible to PEF treatments.^[14,15] The presence of natural antimicrobials can increase the effectiveness of PEF processing of fruit and vegetable juices. Characteristics of fruit and vegetable juices such as pH, water activity, and soluble solids are some of the major factors that can affect the PEF technology in terms of microorganism inactivation. The main components of a PEF system are a high voltage power supply, a treatment chamber, a pulse generator, an energy discharging switch to electrodes and a cooling system to balance temperature rise during treatment.^[16–18]

The PEF process significantly accelerates the juice expression and results in the selective damage of membranes without noticeable changes in the structure of wall components.^[19,20] Above a threshold level of a field strength of 350 V/cm, it was possible to distinguish the individual permeabilized cells by their color. Over 90–95% of the PEF-induced color changes occurred during the first 2–3 min after the electric pulse and the rest after some 20 min. The size and rate of the observed changes were correlated with the severity of PEF, and were further influenced by the pH and conductivity of the solution used in mounting the epidermis, the sampling location on the onion epidermis, and cell size and number.^[21] The advantage of PEF-treatment in extraction applications is related to good pressing efficiency and high juice purity at minimum power consumption.^[22]

Compared with other traditional techniques such as high temperatures and enzymatic treatments, the PEF technology also demonstrated several benefits, such as shorter processing time, lower energy costs (economic benefits), lower treatment temperature, continuous flow, controlling of microbial growth and improvement of polyphenols extraction (Figure 1).^[9,12,18] PEF improves mass transfer rates by electroporation of plant cell membranes improving tissue softness and influencing textural properties.^[23–25] Furthermore, the application of PEF to fruit and vegetable products depending on electric field strength and processing time causes microbial reduction with minimum and/or no detrimental changes in quality of the juices.^[26] New treatment methods involving the use of high-pressure homogenization, ultrasound (US), ultraviolet radiation (UV), and PEF have been used in fruit and vegetable industry.^{[4,17,27–29]} Among these methods, the PEF technique is a new alternative to the traditional production of juices. Therefore, this study summarized the effect of PEF treatment on the physicochemical parameters, color change, bioactive compounds and sensorial properties of some fruit and vegetable juices including apple, carrot, citrus, grape, peach, pomegranate, watermelon and tomato juices. In addition, the main positive effects of the PEF technology on the juices quality and treatment conditions (electric field strength, pulse frequency and time) for some fruit and vegetable products processing are summarized.

Figure 1. Effect of PEF treatment on the properties of fruit and vegetable juices.

Apple juice

Improving the content of bioactive compounds or keeping them at least stable represents a present challenge in food processing. In this context, the emerging and innovative non-thermal technologies such as PEF considered as promising applications for preserving foods or enriching foods with more available bioactive compounds.^[30] Today, there exists a growing interest in the PEF treatment applications for food processing. Table 1 summarizes some studies that evaluating the use of PEF treatment on physicochemical properties of different fruit and vegetable juices. PEF has been reported to be an ideal method to enhance juice production, increasing the content of valuable components and even replacing the enzymatic maceration.

Table 1. Effect of PEF treatment on different characteristics of fruit and vegetable product.

Juice	Processing conditions	Obtained results	Reference
Apple	4 μs, 35 kV/cm and 1200 pps	Minimal variability was observed in pH and no significant changes were detected in acidity, phenolics content and volatile compounds concentration showed statistical significant differences between treatments.	^[Citation31]
Apple	12.5 kV/cm, 76.4 kJ/L, 62 Hz; Flow 27.6 L/h	Polyphenol oxidase and peroxidase activities were largely reduced by HIPEF.	^[Citation32]
Apple	15, 25, and 35 kV/cm, 3 μs, 8.5–65.5 kJ/kg	Maximum polyphenoloxidase deactivation of 48% was achieved when the juices were preheated to 40°C and PEF-treated at 30 kV/cm (100 kJ/kg).	^[Citation33]
Apple	28–40 kV/cm, 25–100 μs	An analysis of the normalized energy densities for all UV/PEF processing conditions indicated a maximized energy efficiency at higher electric field strengths and shorter treatment times.	^[Citation34]
Apple	1–5 kV/cm, n = 30 pulses	Polyphenol contents and antioxidant capacities of the apple juices extracted from PEF-treated apple mash did not significantly differ from the controls.	^[Citation35]
Apple	35 kV/cm, frequency of 1200 PPS	PEF treatment caused a 14.49% reduction in phenolics content, and a significant decrease in volatile compounds in apple juice.	^[Citation31]
Apple	20, 30 and 40 kV/cm; 25, 50 and 100 μs	Highest level of decrease in the enzymatic activity of 71% and 68%, for polyphenoloxidase and peroxidase, respectively by using a combination of preheating to 50°C, and a PEF treatment time of 100 μs at 40 kV/cm, and the kinetic data for the inactivation of both enzymes could be well described using a 1st-order model.	^[Citation36]
Apple	0, 25, 30, and 35 kV/cm for 75 μs with pulse rise time of 2 μs or 0.2 μs	PEF treatment can inactivate the activity of enzymes and keep quality attributes such as vitamin C, total phenols content, antioxidant capacity and color of apple juice.	^[Citation37]
Apple	100 square-wave pulses (1 μs, 15 Hz) at 40 kV/cm	Application of PEF or a combination of UV irradiation and PEF to freshly squeezed apple juice resulted in a similar total microbial reduction compared to the severe heat treatment.	^[Citation38]
Apple	100 μs and 28 kV/cm	Authors did not report any significant color difference (expressed as ΔE) in apple juice treated by PEF.	^[Citation39]
Apple	27–33 kV/cm, 3, 6 or 10 l/h, 200 pulses/s	A significant decrease (33%) in the activity of PPO in freshly squeezed apple juice when heated at 50°C and exposed to PEF at 27 kV/cm for 58.7 μs.	^[Citation40]
Blueberry	0–400 V	During storage of 30 days at 4 °C, Vitamin C decreased by 11.5% and 26.8% after PEF and HTST (high temperature short time) treatments, all treated juices showed to be microbiologically safe, whereas PEF can protect nutritionally related properties and color of blueberry juice samples more efficiently.	^[Citation41]
Carrot	0.25 to 1 kV/cm; 2–40 ms; 1.5 to 576 J/kg	PEF treatment at 1 kV/cm for 2 ms (29 J/kg) increased concentrations of sucrose and β-glucose in 43 and 48%, respectively.	^[Citation42]
Carrot–orange	25, 30, 35, and 40 kV/cm; 30–340 μs	PEF treatment at 25 kV/cm for 280–330 μs extended the half-life of the carrot–orange juice stored at 2°C for 50 days.	^[Citation43]
Carrot and orange (blended)	25 kV/cm and 280 μs, P1; and 25 kV/cm and 330 μs, P2	Reduction in PME activity in addition to the preservation of the sensory characteristics, hydroxymethylfurfural and physicochemical quality similar to the fresh juice.	^[Citation44]
Grape	3 kV/cm; 10 kJ/kg	Extraction of individual anthocyanins was studied showing a selective extraction based on the glucose moieties linked to the anthocyanidins; anthocyanin monoglucosides were better extracted by PEF.	^[Citation45]
Grapefruit	1 kHz, 20 kV/cm for 600 μs	°Brix, pH and acidity showed non-significant effects irrespective of the treatment. But, color values changed significantly after the treatment.	^[Citation46]
Grapefruit	1 kHz; 0, 5, 10, 15, 20 and 25 kV/cm for 600 μs; 80 mL/min	PEF at 25 kV/cm could improve the quality of grapefruit juice. Significant decrease in non-enzymatic browning and viscosity while an increase in cloud value, DPPH, total antioxidant capacity, total phenolics and total carotenoids, while decreases in the activity of micro‐organisms, was also observed with the increase in pulsed electric strength.	^[Citation7]
Grapefruit	20, 30 & 40 kV/cm for 25, 50, 75 and 100 μs	Highest level of inactivation of PME (96.8%) was reported using a combination of preheating to 50°C, and a PEF treatment time of 100 μs at 40 kV/cm.	^[Citation47]
Melon	35 kV/cm for 1709 μs at 193 Hz and 4 μs pulse duration	Melon juice processed by HIPEF and citric acid or cinnamon bark oil exhibited the lower scores in odor, taste, sourness and total acceptance.	^[Citation48]
Opuntia dillenii cactus fruit	56 exponential decay pulses; maximum electric field strength of 3 kV/cm; total specific energy input of 5 kJ/kg fruit mash	Treating fruit mash with a moderate electric field strength increased juice yield and improved juice characteristics. Promisingly, the highest release of isorhamnetin 3-O-rutinoside from fruit’s peel into juice was maximally achieved by PEF.	^[Citation30]
Orange	0–35 kV/cm, 2.2 ms pulse duration time	A combination of PEF treatment at 25 kV/cm and a water bath temperature of 50°C caused 90% inactivation of PME.	^[Citation49]
Orange	40 kV/cm for 97 ms	PEF‐processed juice retained more ascorbic acid, flavor, and color than thermally processed juice.	^[Citation50]
Orange	30 kV/cm for 240 or 480 μs	The average losses of flavor compounds in orange juice processed by 240, 480 μs PEF and heat process were 3.0%, 9.0% and 22.0%, respectively.	^[Citation51]
Orange	35 kV/cm for 59 μs	Packaging material had a considerable effect on the retention of orange juice aroma compounds, color, and vitamin C. PEF-treated orange juice had a shelf life of >16 weeks in glass and polyethylene terephthalate at 4°C.	^[Citation52]
Orange	15, 25 and 35 kV/cm; 100, 400 and 1000 μs; using pulses of 4 μs width at 200 Hz	There were no differences in antioxidant capacity between HIPEF treated and untreated products, whereas heat-treated foods showed lower values of antioxidant capacity.	^[Citation53]
Orange	13.82–25.26 kV/cm, 10.89–51.32 J	Unlike heat treatments, PEF treatments did not cause hydroxyl methyl furfural formation during storage. PEF treatments accomplished ascorbic acid preservation with half-life of 1112 days.	^[Citation54]
Orange	13.82–25.26 kV/cm; 10.89–51.32 J; 1033.9 and 1206.2 μs	The samples treated with PEF had more stable flavonoids and phenolic acids than those treated with the thermal pasteurization. The PEF treated samples had higher sensory scores than the heat treated samples.	^[Citation55]
Orange	35 kV/cm for 1,000 μs; bipolar 4-μs pulses at 200 Hz	Vitamin C retention was outstandingly higher in juice processed by HIPEF fitting recommended daily intake standards throughout 56 days storage at 4°C.	^[Citation56]
Orange, pomelo and lemon	3 kV/cm and 10 kV/cm	The PEF treatment increased the yield of juice obtained after pressing by 25% for orange, 37% for pomelo, and 59% for lemon. Also, significant increase of polyphenols extraction from PEF treated peels was reported.	^[Citation57]
Peach	0, 17, 20, 23, 27 and 30 kV/cm; 50 mL/min flow rate; 3 ms pulse duration, 500 pps; 131 ms	Suggested that the PEF process can successfully be applied to peach nectar with significant amount of microbial inactivation without a significant adverse effect on physical and chemical properties.	^[Citation58]
Peach	0–5 kV/cm; 10–50 pulses of 3 μs (30–150 μs)	PEF-assisted extraction of bioactive compounds from thinned peach fruits using water as a solvent is an alternative to conventional extraction methods which require dried products, large amounts of organic solvents and long extraction times.	^[Citation59]
Peach, apricot and sour cherry	125, 250 and 400 Hz; 24 kV/cm	Initial mean values of the physical properties did not change significantly with the increasing treatment times for sour cherry juice (64%), apricot nectar (60%), and peach nectar (30%). The increasing treatment time did not significantly exert an adverse impact on any of the sensory scores.	^[Citation60]
Peach, apricot and sour cherry	0, 13, 17, 20, 23, 27, 30 and 34 kV/cm; 0, 62, 94, 123, 163, 198 and 218 μs	Light and scanning electron microscopy observations revealed considerable morphological alterations in fungal conidia such as cytoplasmic coagulation, vacuolations, shrinkage and protoplast leakage.	^[Citation61]
Pomegranate	35 and 38 kV/cm, 281 μs at 55°C, flow rate of 100 L/h	PEF processing didn’t significantly impact total phenolic, anthocyanin, and antioxidant compounds in comparison to those in untreated pomegranate juice. PEF processing had significantly less impact on the color of pomegranate juice than thermal processing.	^[Citation62]
Pomegranate	Bench top (7.2 L/h, 35 kV/cm, 72 µs) and pilot scale (100 L/h, 35 kV/cm, 281 µs)	Physicochemical analysis showed that there were no significant differences in total phenolic and antioxidant compounds, pH, and total soluble solid among untreated and PEF treated pomegranate juice.	^[Citation63]
Pomegranate	0, 17, 23, and 30 kV/cm	Total monomeric anthocyanins, total ascorbic acid content and total antioxidant capacity remained constant during PEF treatment. Sensorial properties (flavor, taste, aftertaste, and total acceptance) were not affected by the PEF treatment.	^[Citation64]
Tomato	100 kV/cm; 2 μs width, 0.5 Hz frequency	Enzyme polygalacturonase was unaffected by PEF, but the activity of PME was reduced by 55%.	^[Citation65]
Tomato	0.5–2.5 kV/cm, 0–4000, pulses, 15 μs pulse width	Carotenoid extraction yield increased up to 56.4%. Lycopene extraction increased from 9.84 mg lycopene/100 g to 14.31 mg/100 g tomato residue for a PEF treatment at 1.0 kV/cm for 7.5 ms. The concentration of extracted total phenolic compounds doubled (56.16 mg gallic acid/kg) with a 2 kV/cm, 700 pulses treatment.	^[Citation66]
Tomato	1 kV/cm using 16 monopolar pulses of 4 μs at a frequency of 0.1 Hz	Combination of moderate-intensity PEF and high-intensity PEF treatments could be used not only to produce tomato juices with high carotenoid content but also, to maintain higher the carotenoid content during storage time.	^[Citation67]
Tomato	0.4 to 2.0 kV/cm and number of pulses from 5 to 30	A 44% increase in total polyphenol content was achieved under 30 pulses at 1.2 kV/cm. The hydrophilic antioxidant capacity was also enhanced by 44% applying 18 pulses at 1.2 kV/cm, and the lipophilic antioxidant capacity was increased by 37% under 5 pulses at 1.2 kV/cm.	^[Citation68]
Tomato	35 kV/cm for 1500 µs, using 4 µs bipolar pulses at 100 Hz	High‐intensity PEF treated juice exhibited higher levels of compounds contributing to tomato aroma than untreated and heat‐treated juices throughout storage.	^[Citation69]
Tomato	35 kV/cm for 1500 μs in bipolar 4-μs pulses at 100 Hz, with an energy density of 8269 kJ/L	HIPEF treated tomato juice maintained higher lycopene and vitamin C content than the thermally treated juices during the storage time.	^[Citation70]
Tomato	40 kV/cm for 57 μs	PEF processed tomato juice had significantly lower nonenzymatic browning and higher redness than thermally processed or control juice. Sensory evaluations indicated that the flavor of PEF processed juices was preferred to that of thermally processed tomato juices.	^[Citation71]
Watermelon	35 kV/cm for 1727 μs applying 4-μs pulses at 188 Hz in bipolar mode	Use of HIPEF technology could be an alternative to thermal treatments and could contribute to better maintain valuable attributes of watermelon juice.	^[Citation72]
Watermelon	35 kV/cm for 1682 μs at 193 Hz and 4 μs pulse duration	Watermelon juice processed by HIPEF and citric acid or cinnamon bark oil displayed the lower rating in taste, sourness and overall by the panelists.	^[Citation48]
White grape	0.25–1 kV/cm	PEF treatment results in increase of the juice yield from 49–54% to 76–78% at 45 min of pressing.	^[Citation73]
White grape	0.4 kV/cm	PEF treatment accelerated development of grape deformation. Statistical analysis showed no significant effect of PEF treatment on turbidity and content of polyphenols for the constant pressure regime.	^[Citation74]

The effects of PEF on pH, soluble solids, acidity, polyphenoloxidase and peroxidase activities, Vitamin C and polyphenolic contents, and color of apple juice were studied by some researchers.[31,34–36,38,75,76] As an alternative to thermal pasteurization, PEF process were used by Schilling, et al.[^33] to apple juices on laboratory and pilot plant scale. The authors reported that the maximum polyphenoloxidase deactivation of 48% was achieved when the juices were preheated to 40°C and PEF-treated at 30 kV/cm (100 kJ/kg). Therefore, minimally processed apple juices resulted from PEF processing, when used without additional conventional thermal preservation. Aguilar-Rosas, et al.^[31] reported that PEF processing (4 μs, 35 kV/cm and 1200 pps) retained most of the volatile compounds responsible for color and flavor of apple juice better than heat treatment and caused 7% and 8.4% losses of hexanal and hexyl acetate, respectively, unlike heat treatment which caused their complete losses. In another study, the effects of PEF process on physicochemical and sensory properties of apple juice was investigated .^[37] The authors reported that the apparent viscosity and consistency index (K) of apple juice decreased while the flow behavior index (n) increased with increasing the electric field strength and apple juice treated at 2 μs pulse rise time had significantly higher apparent viscosity than treated at 0.2 μs pulse rise time. The influence of PEF processing on the physicochemical attributes of fruit and vegetable product was reported in Table 2. PEF-treated apple juice had a significantly higher L* (lightness) and b* (yellowness) values than the controlled sample.^[37]

Table 2. Effect of PEF processing on the physicochemical attributes of fruit and vegetable product.

Juice	Treatment	pH	°Brix	titratable acidity (%)	L*	a*	b*	Chroma	Reference
Apple	Untreated	3.32	13.08	0.47	50.62	0.24	32.36	32.37	^[Citation32]
	Treated	3.45	12.78	0.45	57.53	2.65	31.74	31.85
Apple	Untreated	-	-	-	44.16	−2.75	12.84	-	^[Citation37]
	Treated	-	-	-	51.86	−2.97	16.12	-
Apple	Untreated	3.69	12.8	-	25.40	−0.72	3.99	-	^[Citation38]
	Treated	3.66	12.9	-	27.77	−0.70	3.53	-
Apple	Untreated	3.75	-	-	37.28	9.22	6.72	-	^[Citation76]
	Treated	3.73	-	-	36.70	9.63	6.73	-
Apricot	Untreated	3.21	11.22	1.50	34.10	10.93	35.39	-	^[Citation60]
	Treated	3.18	11.27	1.55	33.84	10.67	34.98	-
Blueberry	Untreated	-	1.57	2.08	42.25	40.05	25.73	-	^[Citation41]
	Treated	-	1.57	2.08	41.33	40.20	25.89	-
Carrot and orange (blended)	Untreated	3.83	9.5	0.568	62.80	-	-	81.01	^[Citation44]
	Treated	3.86	10.2	0.599	63.08	-	-	80.12
Grapefruit	Untreated	2.96	8.9	-	-	-	-	-	^[Citation77]
	Treated	2.96	8.9	-	-	-	-	-
Grapefruit	Untreated	4.51	10.0	0.11	3.88	1.94	2.21	-	^[Citation46]
	Treated	4.54	10.7	0.11	3.89	1.95	2.23	-
Melon	Untreated	6.12	11.7	-	-	-	-	-	^[Citation48]
	Treated	6.09	11.2	-	-	-	-	-
Opuntia dillenii cactus fruit	Untreated	3.67	10.5	-	22.2	3.2	−1.6	3.4	^[Citation30]
	Treated	3.65	10.7	-	22.3	2.9	−1.7	3.3
Orange	Untreated	3.65	10.6	-	-	-	-	-	^[Citation77]
	Treated	3.65	10.6	-	-	-	-	-
Peach	Untreated	3.9	10.0	0.19	41.30	4.94	29.91	-	^[Citation58]
	Treated	3.9	10.0	0.19	40.29	3.93	28.04	-
Peach	Untreated	3.22	10.35	0.18	41.67	4.48	29.87	-	^[Citation60]
	Treated	3.16	10.35	0.19	40.87	4.31	28.82	-
Pomegranate	Untreated	3.85	16.57	-	-	-	-	-	^[Citation63]
	Treated	3.86	16.50	-	-	-	-	-
Pomegranate	Untreated	3.28	14.15	0.17	17.06	30.22	15.99	-	^[Citation65]
	Treated	3.25	14.1	0.19	19.15	35.35	18.92	-
Sour cherry	Untreated	3.18	13.45	0.25	1.80	2.75	0.85	-	^[Citation60]
	Treated	3.19	13.45	0.26	0.91	3.11	0.56	-
Tomato	Untreated	4.24	3.8	-	22.8	-	-	-	^[Citation67]
	Treated	4.41	3.9	-	24.8	-	-	-
Tangerine	Untreated	2.95	10.2	-	-	-	-	-	^[Citation77]
	Treated	3.00	10.2	-	-	-	-	-
Watermelon	Untreated	5.76	10.6	-	-	-	-	-	^[Citation48]
	Treated	5.77	10.7	-	-	-	-	-

Carrot

Carrot, a nutritionally vegetable with a wide range of color, is rich in β-carotene, thiamine, vitamin B-complex, riboflavin and minerals. It is well known for their high content of carotenoids, which work as precursors to vitamin A.^[78,79] The effects of PEF on ascorbic acid, color, vitamins, riboflavin and polyphenolic contents of carrot juice were studied by some researchers.^{[42,43,80–83]} The PEF treatment was shown to be useful for preserving organoleptic properties, ascorbic acid content and nutritive values of juices as compared with conventional thermal treatment. Ascorbic acid stability during refrigerated storage of carrot–orange juice treated by high PEF (25–40 kV/cm and 30–340 μs) and comparison with pasteurized juice was studied.^[43] The shelf life of the carrot–orange juice treated by pulses at 25 kV/cm for 280 μs and 330 μs was compared with a heat-treated juice (98°C, 21 s) kept in refrigerated storage at 2 and 10°C. The remaining concentration of ascorbic acid in the pasteurized carrot–orange juice was 83%, whereas in the PEF-treated juice it was 90%. Aguiló-Aguayo, et al.^[80] reported an increment in the falcarinol (FaOH), falcarindiol (FaDOH) and falcarindiol-3-acetate (FaDOAc) between 67 and 88% after applying 4 kV/cm with 100 number of pulses of 10 μs at 10 Hz. Similarly, the pre-treatment with PEF at 0.25 kV/cm for 6 ms or 40 ms on carrot puree induced an enhancement in the FaDOH and FaDOAc contents by 3-fold or in FaOH up to 1.9-fold, respectively.^[42] In another study, the effect of different PEF process intensities and heat treatment on quality characteristics of blended orange and carrot juice were investigated by Rivas, et al. .^[44] The authors reported that the hydroxymethylfurfural, L* and C* (chrome) color parameters did not vary with any of the treatments. Sensory attribute of the PEF-treated juice were more similar to the untreated juice than the pasteurized juice.

Citrus juices

There are many published works that study the effect of PEF on physicochemical attributes, polyphenols extraction, microorganisms and shelf-life of citrus juice.^{[18,46,50,51,57,77,84,85]} PEF‐processed orange juice retained more ascorbic acid, flavor and color than thermally processed juice.^[50] The effect of PEF processing on the organoleptic characteristics of fruit and vegetable juices was reported in Table 3.

Table 3. Effect of PEF processing on the organoleptic characteristics of fruit and vegetable juices.

Juice	Treatment	Appearance	Flavor	Odor	Color	Taste	Texture	Sourness	Overall	Reference
Apricot	Untreated	-	5.73	-	6.26	6.84	-	6.39	6.47	^[Citation60]
	Treated	-	6.11	-	6.61	6.73	-	6.16	6.91
Melon	Untreated	-	-	7.7	7.3	7.9	-	7.1	7.7	^[Citation48]
	Treated	-	-	6.9	7.3	6.9	-	7.1	7.2
Orange	Untreated	6.8	-	-	7.0	7.8	7.3	-	7.4	^[Citation50]
	Treated	6.2	-	-	6.8	5.9	6.0	-	6.1
Orange	Untreated	-	-	8.6	9.0	-	-	-	8.7	^[Citation55]
	Treated	-	-	7.9	8.3	-	-	-	7.5
Peach	Untreated	-	5.75	-	6.60	5.00	-	6.45	5.62	^[Citation60]
	Treated	-	6.32	-	6.37	4.70	-	6.25	5.70
Pomegranate	Untreated	-	-	-	7.8	6.4	-	6.0	7.0	^[Citation65]
	Treated	-	-	-	6.4	6.0	-	4.0	5.8
Sour cherry	Untreated	-	5.31	-	5.84	5.60	-	5.55	6.47	^[Citation60]
	Treated	-	5.75	-	5.98	5.95	-	5.93	6.91
Watermelon	Untreated	-		6.7	8.3	7.2	-	7.1	7.3	^[Citation48]
	Treated	-		6.7	8.1	7.6	-	7.4	7.2

Physicochemical parameters (pH, °Brix, electric conductivity and color) and sensory properties of PEF treated (28 kV/cm with 50 pulses) orange, grapefruit and tangerine juices was investigated by Hartyáni, et al.^[77] The authors reported that the pH values of orange and grapefruit juice stayed quite stable in the values of control and PEF-treated samples. Also, the color of the citrus juices, which are treated with PEF show stronger tendency to the yellow direction and lower tendency toward the red color. In addition, the quantity of ethyl-esters did not decrease, respectively the volatile aroma components of orange juice, namely, ethyl butyrate, linalool were not lost significantly as the effect of the PEF treatment. Elez-Martínez, et al.^[56] reported the pH and acidity of heated (90°C, 1 min) and HIPEF treated (35 kV/cm, 1000 μs, with 4 μs bipolar pulses at 200 Hz) orange juice during the storage of 56 days at both 4 and 22°C. HIPEF treated orange juice retained better color than heat-pasteurized juice throughout storage but no differences were found between treatments in pH, acidity and °Brix. Vitamin C retention was outstandingly higher in orange juice processed by HIPEF fitting recommended daily intake standards throughout 56 days storage at 4°C, whereas heat-processed juice exhibited a poor vitamin C retention beyond 14 days storage (25–43%). The effects of PEF on quality parameters of grapefruit juice was studied.^[46] It was observed that changes occur in acidity, °Brix, pH, electrical conductivity and color values of grapefruit juice after treatment were not much significant. However, noteworthy increase in cloud value and non-enzymatic browning were observed in all samples after combined treatment of sonication and PEF.

Enzyme inactivation may be caused by PEF induced conformational changes in the tertiary structure of the enzyme.^[47] PME causes cloud loss in citrus juices by deesterification of the pectin. Decreasing the PME action could limit the degradation of pectin substances. Thus, concomitant losses in viscosity and cloud stability could be diminished.^[72] Inactivation of PME in orange juice by PEF was examined by Yeom, et al.^[49] Increase of electric field strength caused a significant inactivation of PME with increase in orange juice temperature. Also, higher activation energy is required to inactivate PME than that required to inactivate microorganisms by PEF treatment. Jia, et al.^[51] reported that PEF-processed orange juice resulted in no significant change in the initial decanal and octanal concentrations, whereas heat treatment (90°C for 3 min) decreased the initial decanal and octanal concentrations by 40 and 9.9 %, respectively. Elez-Martínez and Martín-Belloso^[53] studied the effects of HIPEF processing conditions on vitamin C and antioxidant capacity of orange juice and gazpacho, a cold vegetable soup. The authors reported that HIPEF processing slightly decreased the vitamin C concentration compared to the negative impact of conventional thermal pasteurization on orange juice. They investigated the impact of field strengths of 15, 25, and 35 kV/cm aiming at preserving the orange juice, while in the present study PEF was applied in a lower field strength (3 kV/cm) for cell disintegration aiming only at increasing yield of juice and bioactive substances. However, some researchers reported that vitamin C content was not changed in orange juice when treated with field strength of 35 kV/cm^[52] or 40 kV/cm.^[50]

Grape juice

The mechanism of PEF action is that it induces electroporation of cell membranes, increasing their permeability and the release of valuable compounds.^[86] PEF has also shown to enhance the extraction of other bioactive compounds such as anthocyanins from purple-fleshed grapes.^[45] A number of authors have studied the evolution of quality factors in grape juice after PEF treatment. In white grapes, a field strength of 750 V/cm during PEF treatment increased the juice yields from 49% to 54%, obtained from untreated grapes, to 73–78% by pressing for 45 min at constant pressure of 5 bar.^[73] The authors reported that PEF pre-treatment resulted in an increase of 25% of juice yield being more efficient than an intermediate PEF treatment. In another study, treating grapes with an electric field strength of 400 v/cm prior to pressing the fruits increased juice yield from 67% to 75%.^[74]

Peach juice

In the L*a*b* space, L* is lightness/darkness that ranges from 0 to 100, a* is redness/greenness that ranges from −120 to 120 and b* is yellowness/blueness that ranges from −120 to 120.^[87,88] Applicability of PEF to process peach nectar as a function of electric field strength and treatment time with respect to measurement of physical and chemical properties of peach nectar was evaluated.^[58] Based on the results reported in this study, no significant difference was detected in terms of pH, °Brix, titration acidity, conductivity, color parameters (L*, a* and b*), nonenzymatic browning index, metal ion concentration, ascorbic acid content and beta carotene concentration between control and PEF‐treated peach nectar samples. So, PEF can successfully be applied to peach nectar without adverse effect on physicochemical properties. Redondo, et al.^[59] PEF process as a green technology for the extraction of bioactive compounds from thinned peach by-products. The authors reported that the extraction yield of bioactive compounds from thinned fruits using PEF is dependent on the electric field strength, extraction temperature and methanol percentage, the solvent being the most important factor.

Pomegranate juice

Evaluation of bioactive compounds, physicochemical properties and consumer acceptance of pomegranate juice processed in a commercial scale PEF system was investigated by Guo, et al.^[62] This study demonstrated that PEF technology extended microbial shelf-life and preserved the major quality and nutritional characteristics of pomegranate juice, and hence, is technically feasible for commercialization in the juice industry. PEF processing had significantly less impact on the color of pomegranate juice than thermal processing. In another study, Jin, et al.^[63] used PEF processing systems for extending microbiological shelf-life of pomegranate juice. The authors reported that the pomegranate juice treated with PEF alone had a shelf-life of 21 days (bench scale) and over 84 days (pilot scale). In addition, the physicochemical analysis showed that there were no significant differences in total phenolic and antioxidant compounds, pH, and Brix among untreated, PEF treated, and PEF and antimicrobial coating treated samples. However, the use of a large scale PEF processing system and the combination of antimicrobial packaging provide juice manufacturers an innovate approach for enhancing the safety and increasing the shelf-life of pomegranate juice products. Evrendilek^[64] studied the effect of PEF and heat treatment on quality and sensory properties and microbial inactivation of pomegranate juice. The author reported that the sensory properties of pomegranate juice were not significantly affected by PEF alone and PEF + mild heat treatments.

Watermelon juice

Among the non-thermal processes, PEF is the most widely used for watermelon juice processing and also showed most acceptable results.^[17] Non-thermal cell disintegration using PEF is a promising preservation technology for liquid or semi-liquid, viscous food products (such as fruit and vegetable juices).^[30] The effects of HIPEF processing (35 kV/cm for 1727 μs applying 4-μs pulses at 188 Hz in bipolar mode) on color, viscosity and related enzymes in watermelon juice were evaluated during 56 days of storage and compared to thermal treatments (90°C for 60 s or 30 s).^[72] HIPEF-treated juice maintained brighter red color than thermally treated juices along the storage time. In addition, the application of HIPEF as well as heat at 90°C for 60 s led to juices with higher viscosity than those untreated for 56 days of storage. Mosqueda-Melgar, et al.^[48] also studied PEF processing of watermelon juice combined with natural antimicrobials (citric acid and cinnamon bark oil). The authors found that PEF had a significant effect on taste and odor of the watermelon juice. The microbiological quality and safety of these fruit juices by combining HIPEF and citric acid or cinnamon bark oil were ensured.

Tomato

The tomato processing industry strives to maximize product yield, keep energy costs and waste effluents to a minimum while maintaining high product quality. PEF processing enhances plant cell permeability through electroporation and could be applied in tomato processing to facilitate peeling, increase juice yields and enhance valorization of tomato waste.^[66]

In tomato juices, studies have proven that PEF treatments are efficient enough to destroy microorganisms in tomato juices without greatly affecting their nutritional and sensory properties in comparison to heat pasteurization.^{[69,71,89,90]} Giner, et al.^[91] reported a reduction of 93.8% of PME from tomato juice after applying a HIPEF treatment set up at 24 kV/cm with 400 0.02-ms pulses.

Increases in lycopene content as well as in the antioxidant capacity of Moderate-intensity PEF treated tomato fruit were observed 24 h after treatments, depending on the electric field strength (0.4–2 kV/cm) and number of pulses (5–30) applied.^[68] In another study, the effect of PEF on the carotenoid content of tomato juices was studied.^[67] Moderate-intensity PEF treatment of tomatoes increased the content of carotenoid compounds in tomato juices and an enhancement of 63–65% in 15-cis-lycopene was observed in juices prepared with moderate-intensity PEF treated tomatoes. Many studies showed that the antimicrobial effect of PEF is related to the electric field strength, numbers of pulses applied, temperature and added antimicrobials. Inactivation of naturally occurring microorganisms in tomato juice using PEF with and without antimicrobials was studied by Nguyen and Mittal.^[65] At field strength of 80 kV/cm, 20 pulses, 50°C, and in the presence of nisin (100 U/mL), there was about 4.4 log reductions in microbial counts. There was no Vitamin C decreases due to the treatment.

Conclusion

Consumer’s demand for high quality healthy foods rich in natural bioactive compounds such as vitamins, phenolic compounds, pigments and dietary fibers is drastically increasing. Treatment of fruit and vegetable juices with the PEF processing is one of the non-thermal alternative technologies with the great potential for microbial and enzyme inactivation, shelf-life extension, and preservation of nutritional and sensorial properties. PEF treated juices maintained higher vitamins content than the thermally treated juices during the storage time. Sensory evaluations indicated that the flavor and taste of PEF processed juices were preferred to that of thermally processed juices. The impact of pulses of high voltage on the permeability of biological membranes leads to the inactivation of microorganisms, consequently improving shelf life and quality of the preserved juices. Generally, PEF pretreatments incorporated to industrial fruit and vegetable juices processing lead to declined energy demand and increased productivity.